Its tricky to shop for one person, for a couple of days with fairly limited storage and no refrigeration. You can just take a spoon and a p-38, eat out of cans. I have never done that personally, but I have seen people who do it. Like some others here I keep an emergency stash, usually some couscous as it packs dense. Just enough to fill me up once or twice. Also, I carry a light tupperware container for leftovers from restaurants or what I cook.
A lot for me depends on what kind of stove I have decided to bring on the trip. If its an light weight alcohol stove with one small pot, I will pick up ramen packs, Knorr side dish stuff(needs to be cooked longer, but if you use a pot cozy method it can work). Usually buy some packs of tuna, chicken, or other protein to mix in with whatever one pot meal I have. On light trips I pick up a few candy bars(payday mostly)at gas stations or the like.
Bread, usually an uncut loaf, whether its a light weight trip or not.
When traveling with a full load of four panniers, with a gas stove, larger pot and a cheap light teflon skillet, my larder gets bigger, but what I have depends on the temperatures. A packet of mixed veggies from the salad aisle, cheese, sausage. enough for a couple of days. Recently I have started carrying eggs, just use the carton they are sold in, with a cardboard box for support. A couple of packs of spam singles so I can have spam and eggs in the morning(Personally dislike oatmeal and the like). Even baked biscuits using the canned stuff purchased the night before.
One thing I like to take is olive oil. Started buying it in the spray can since there is little chance of it leaking. stir fry veggies with noodles is fast to make.

things I might buy at a store to eat in a day or so. not all at once, but just in general.
Bell peppers, a small onion and sausage
A bag of salad mix and croutons.
can of corn beef hash and stock up on eggs.
block of cheese, and loaf of french bread, small packet of olives(if its in a bag, I try to avoid jars)
Hot dogs(they will keep longer than you think if kept fairly cool and in a clean container)and sauerkraut or can of beans.

If there is no where to shop but dollar generals, I will pick up pop tarts, instant cups of soup, crackers and Vienna sausages. If there is nothing but gas stations, then if they have a fried food counter stock up on chicken strips and whatnot.

Eating well also depends on your metabolism. I like to eat well, but can operate on nothing but ramen or peanut butter for days, and have enough fat reserves to go hungry for a night without dying. everyone is different, so its a personal thing based on experience and physical needs.

Came back to edit, I always take hot sauce. Whatever is the hottest that I can find. Usually take a brand with me simply called "pain". a little goes a long way.